Isotope effects on the metabolism and pulmonary toxicity of butylated hydroxytoluene in mice by deuteration of the 4-methyl group

THE USE OF DEUTERIUM ISOTOPE EFFECTS TO PROBE THE ACTIVE SITE PROPERTIES, MECHANISM OF CYTOCHROME P450-CATALYZED REACTIONS, AND MECHANISMS OF METABOLICALLY DEPENDENT TOXICITY

Critical elements from studies that have led to our current understanding of the factors that cause the observed primary deuterium isotope effect, (kH/kD)obs, of most enzymatically mediated reactions to be much smaller than the "true" or intrinsic primary deuterium isotope effect, kH/kD, for the reaction are presented. This new understanding has provided a unique and powerful tool for probing the catalytic and active site properties of enzymes, particularly the cytochromes P450 (P450). Examples are presented that illustrate how the technique has been used to determine kH/kD, and properties such as the catalytic nature of the reactive oxenoid intermediate, prochiral selectivity, the chemical and enzymatic mechanisms of cytochrome P450-catalyzed reactions, and the relative active site size of different P450 isoforms. Examples are also presented of how deuterium isotope effects have been used to probe mechanisms of the formation of reactive metabolites that can cause toxic effects.

Protein covalent binding of maxipost through a cytochrome P450-mediated ortho-quinone methide intermediate in rats

(3S)-(+)-(5-Chloro-2-methoxyphenyl)-1,3-dihydro-3-fluoro-6-(trifluoromethyl)-2H-indole-2-one) (MaxiPost, BMS-204352) is a potent and specific opener for maxi-K channels and has potential to prevent and treat ischemic stroke. Following single intravenous doses of [14C]BMS-204352 to rats, only 10 to 12% of radioactivity was extractable from plasma with organic solvents. The unextractable radioactivity remained associated with the proteins (mostly albumin) after SDS-polyacrylamide gel electrophoresis or dialysis. Following acid hydrolysis in 6 M HCl for 24 h at 110 degrees C from plasma proteins collected from nine rats dosed with [14C]BMS-204352, one major radioactive product was isolated and identified as a lysine-adduct of des-fluoro des-O-methyl BMS-204352 by liquid chromatography/mass spectrometry and NMR analyses as well as by comparison with the synthetic analog, lysine-adduct of des-fluoro BMS-204352 (BMS-349821). The covalent binding of BMS-204352 results from the displacement of the ring-fluorine atom of des-O-methyl BMS-204352 with the epsilon-amino group of a lysine residue. Microsomal incubations of [14C]BMS-204352 resulted in low levels of covalent binding of radioactivity to proteins. This in vitro covalent binding required cytochrome P450-reductase cofactor NADPH and was attenuated by glutathione. P4503A inhibitors ketoconazole and troleadomycin selectively prevented the covalent binding in vitro. Based on these observations, a two-step bioactivation process for the protein covalent binding of BMS-204352 was postulated: 1) P4503A-mediated O-demethylation leading to spontaneous release of HF and the formation of an ortho-quinone methide reactive metabolite and 2) nucleophilic addition of the epsilon-amino group of protein lysine residue(s) in protein to form des-fluoro des-O-methyl BMS-204352 lysine adduct.

Final report on the safety assessment of BHT(1)

BHT is the recognized name in the cosmetics industry for butylated hydroxytoluene. BHT is used in a wide range of cosmetic formulations as an antioxidant at concentrations from 0.0002% to 0.5%. BHT does penetrate the skin, but the relatively low amount absorbed remains primarily in the skin. Oral studies demonstrate that BHT is metabolized. The major metabolites appear as the carboxylic acid of BHT and its glucuronide in urine. At acute doses of 0.5 to 1.0 g/kg, some renal and hepatic damage was seen in male rats. Short-term repeated exposure to comparable doses produced hepatic toxic effects in male and female rats. Subchronic feeding and intraperitoneal studies in rats with BHT at lower doses produced increased liver weight, and decreased activity of several hepatic enzymes. In addition to liver and kidney effects, BHT applied to the skin was associated with toxic effects in lung tissue. BHT was not a reproductive or developmental toxin in animals. BHT has been found to enhance and to inhibit the humoral immune response in animals. BHT itself was not generally considered genotoxic, although it did modify the genotoxicity of other agents. BHT has been associated with hepatocellular and pulmonary adenomas in animals, but was not considered carcinogenic and actually was associated with a decreased incidence of neoplasms. BHT has been shown to have tumor promotion effects, to be anticarcinogenic, and to have no effect on other carcinogenic agents, depending on the target organ, exposure parameters, the carcinogen, and the animal tested. Various mechanism studies suggested that BHT toxicity is related to an electrophillic metabolite. In a predictive clinical test, 100% BHT was a mild irritant and a moderate sensitizer. In provocative skin tests, BHT (in the 1% to 2% concentration range) produced positive reactions in a small number of patients. Clinical testing did not find any depigmentation associated with dermal exposure to BHT, although a few case reports of depigmentation were found. The Cosmetic Ingredient Review Expert Panel recognized that oral exposure to BHT was associated with toxic effects in some studies and was negative in others. BHT applied to the skin, however, appears to remain in the skin or pass through only slowly and does not produce systemic exposures to BHT or its metabolites seen with oral exposures. Although there were only limited studies that evaluated the effect of BHT on the skin, the available studies, along with the case literature, demonstrate no significant irritation, sensitization, or photosensitization. Recognizing the low concentration at which this ingredient is currently used in cosmetic formulations, it was concluded that BHT is safe as used in cosmetic formulations.

Isotopic effect study of propofol deuteration on the metabolism, activity, and toxicity of the anesthetic

The use of isotopic substitution to delay the oxidative metabolism of the anesthetic propofol 1 was studied. The aromatic hydrogens of propofol 1 were replaced by deuterium to produce the mono- and trideuterated derivatives 4 and 5. In vitro metabolic studies on human hepatic microsomes showed no isotopic effect in the para hydroxylation of propofol, and 1, 4, and 5 display similar hypnotic activity and toxicity in mice.

Studies using structural analogs and inbred strain differences to support a role for quinone methide metabolites of butylated hydroxytoluene (BHT) in mouse lung tumor promotion

Chronic treatment of BALB and GRS mice with BHT (2,6-di-tert-butyl-4-methylphenol) following a single urethane injection increases lung tumor multiplicity, but this does not occur in CXB4 mice. Previous data suggest that promotion requires the conversion of BHT to a tert-butyl-hydroxylated metabolite (BHTOH) in lung and the subsequent oxidation of this species to an electrophilic quinone methide. To obtain additional evidence for the importance of quinone methide formation, structural analogs that form less reactive quinone methides were tested and found to lack promoting activity in BHT-responsive mice. The possibility that promotion-unresponsive strains are unable to form BHTOH was tested by substituting this compound for BHT in the promotion protocol using CXB4 mice. No promotion occurred, and in-vitro work demonstrated that CXB4 mice are, in fact, capable of producing BHTOH and its quinone methide, albeit in smaller quantities. Incubations with BALB lung microsomes and radiolabeled substrates confirmed that more covalent binding to protein occurs with BHTOH than with BHT and, in addition, BHTOH quinone methide is considerably more toxic to mouse lung epithelial cells than BHT quinone methide. These data are consistent with the hypothesis that a two-step oxidation process, i.e. hydroxylation and quinone methide formation, is required for the promotion of mouse lung tumors by BHT.

Oxidation of eugenol by purified human term placental peroxidase

The oxidation of eugenol by purified human term placental peroxidase (HTPP) was examined. Spectral analyses indicated that, similar to horseradish peroxidase, HTPP is capable of catalyzing the oxidation of eugenol. The accumulated stable product in the reaction medium due to eugenol oxidation by HTPP was tentatively identified as quinone methide of eugenol (EQM). The EQM formation exhibited a pH optimum of 8.0 and was dependent on incubation time, amount of HTPP and the concentration of both eugenol and hydrogen peroxide. The specific activity of approx 2.8 micromoles of EQM/min/mg protein was observed with different preparations of HTPP. The EQM formation was significantly suppressed by glutathione and ascorbic acid. The classical peroxidase inhibitors viz. potassium cyanide and sodium azide blocked the reaction in a concentration manner. Collectively, the results suggest that eugenol may undergo peroxidative metabolism in human placenta.

Transcriptional activity of quinone methides derived from the tumor promoter butylated hydroxytoluene in HepG2 cells

Butylated hydroxytoluene (BHT) is a pulmonary toxin and tumor promoter in mice presumably due to the formation of two quinone methides (QMs) that alkylate cellular nucleophiles. The activation of stress genes by these electrophilic metabolites was investigated with an assay system consisting of 14 recombinant cell lines derived from the human hepatoma line HepG2, each carrying a unique promoter or response element construct fused to the reporter gene for chloramphenicol acetyl transferase (CAT). The largest responses to QMs occurred in cells containing either the metallothionein IIA, glutathione S-transferase Ya, or 70 kDa heat shock protein promoter, or the xenobiotic response element. The other cell lines exhibited only small or no effects. These results are consistent with transcriptional activities reported for several other electrophiles known to undergo covalent interactions with proteins.

Influence of quinone methide reactivity on the alkylation of thiol and amino groups in proteins: studies utilizing amino acid and peptide models

Quinone methides (QMs) are electrophiles formed in several biological processes including direct oxidations of 4-alkylphenols by cytochromes P450. These species may be responsible for the adverse effects of certain phenolic compounds through protein alkylation, but little information is available concerning specific targets or the resulting mechanisms of cell injury. The present goal was to determine the most likely sites of adduct formation among competing protein nucleophiles utilizing QMs of varying electrophilicity. Reactions of poorly reactive, moderately reactive, and highly reactive QMs, 2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone (BHT-QM), 6-tert-butyl-2-(2'-hydroxyl-1',1'-dimethylethyl)-4-methylene- 2,5-cyclohexadienone (BHTOH-QM), and 2-tert-butyl-6-methyl-4-methylene-2,5-cyclohexadienone (BDMP-QM), respectively, were investigated in aqueous solutions with nucleophilic amino acids. Each QM rapidly formed a thioether derivative of cysteine with little or no competition from the addition of water (hydration). The alpha-amino groups were the primary sites of alkylation for all other amino acids examined including lysine, histidine, tyrosine, and serine, and the pseudo-first order rates were 5 to 8-fold greater than the rates of hydration. Alkylation of the side chain nitrogens of lysine and histidine occurred at about one-fourth the rate of hydration for BDMP-QM, but no reaction was detectable for BHT-QM and no reactions occurred between QMs and amino acid hydroxyl groups. The results indicate that, based on chemical reactivity, peptide alkylation should occur in the order cysteine thiol > N-terminal amino > N epsilon-lysine = NIm-histidine, with side chain modifications occurring only with the more electrophilic QMs. Reactions of QMs with the tripeptide Gly-His-Lys confirmed the results with amino acids as N alpha-glycine alkylation predominated, but side chain adducts also formed with BHTOH-QM and BDMP-QM. Human hemoglobin was treated with QMs, hydrolyzed, and assayed by HPLC-thermospray mass spectrometry. This work revealed that N epsilon-lysine was the main alkylation site, emphasizing the importance of factors, in addition to chemical reactivity, which influence protein modification by electrophiles.

Studies on the mechanism of hepatotoxicity of 4-methylphenol (p-cresol): effects of deuterium labeling and ring substitution

We recently observed that 4-methylphenol (p-cresol) is toxic to rat liver tissue slices. A possible mechanism involves biotransformation of 4-methylphenol to a reactive quinone methide intermediate which covalently binds to cellular macromolecules and elicits cytotoxicity. In order to obtain further evidence for this proposed mechanism, we studied the effects of deuterium-labeled 4-methylphenol (4-[alpha, alpha, alpha-d3]-methylphenol), and the presence of various ring substituents, on the metabolism and toxicity of 4-methylphenol in precision cut liver slices prepared from male Sprague-Dawley rats. Deuterium-labeled 4-methylphenol was significantly less toxic than the parent compound in rat liver slices (LC50 = 3.36 vs. 1.31 mM, respectively). In addition, the deuterium-labeled compound was metabolized to a reactive intermediate (measured as glutathione conjugate formation) at a slower rate than that of 4-methylphenol in both liver slices and liver microsomal incubations. The presence of electron withdrawing substituents (2-chloro or 2-bromo) markedly enhanced both metabolism and toxicity, with the exception of 2,6-dibromocresol, which was similar to cresol in terms of rate of metabolism and toxicity. Conversely, the presence of electron donating substituents (2-methoxy, 2-methyl or 2,6-dimethyl) diminished metabolism and toxicity. In addition, methylation of the hydroxyl group to form 4-methylanisole, greatly reduced toxicity. These results support the hypothesis that the toxicity of 4-methylphenol is dependent on the formation of a reactive quinone methide intermediate.

Biological and toxicological consequences of quinone methide formation

Quinone methides are a class of reactive, electrophilic compounds which are capable of alkylating cellular macromolecules. They are formed during xenobiotic biotransformation reactions and are hypothesized to mediate the toxicity of a large number of quinone antitumor drugs as well as several alkylphenols. In addition, oxidation of specific endogenous alkylphenols (e.g. coniferyl alcohol) and alkylcatechols (e.g. N-acetyldopamine, dopa) to quinone methides plays an important role in the synthesis of several complex plant and animal polymers, including lignin, cuticle and melanin. The role of quinone methides in these various processes is reviewed.

Relationship of membrane fluidity, chemoprotection, and the intrinsic toxicity of butylated hydroxytoluene

In isolated rat hepatocytes, many chemicals elicit toxicity which is inhibitable by antioxidants such as butylated hydroxytoluene (BHT). Although BHT protection is evident at concentrations of less than about 50 nmol/mg protein, higher concentrations exhibit intrinsic concentration-dependent toxicity, which involves mitochondrial dysfunction. We evaluated the possibility that both chemoprotection and intrinsic toxicity could be explained by a common mechanism involving alterations in the physical properties of cellular membranes. In the red blood cell (RBC) osmotic fragility assay, BHT at less than 60 nmol/mg protein protected against osmotic fragility; however, BHT at higher concentrations enhanced osmotic fragility such that total osmolysis occurred at 135 nmol/mg. The BHT-mediated alterations in osmotic fragility correlated with changes in membrane fluidity, determined by fluorescence polarization of the hydrophobic probe 1,6-diphenyl-1,3,5-hexatriene. Protection from osmolysis correlated with decreased fluidity, while enhanced RBC fragility correlated with increased fluidity. In rat hepatocyte suspensions, high BHT concentrations also permeabilized the plasma and mitochondrial membranes to enzyme leakage, and these effects were accompanied by enhanced membrane fluidity. Although other mechanisms may be operative, alterations in membrane fluidity appear to be, in part, responsible for the observed chemoprotective effects at low concentrations, and intrinsic toxicity at higher concentrations of BHT.

Relationship between the metabolism of butylated hydroxytoluene (BHT) and lung tumor promotion in mice

The widely used antioxidant butylated hydroxytoluene (BHT, 2,6-di-tert-butyl-4-methylphenol) produces acute pulmonary toxicity in mice, and also enhances the multiplicity of lung tumors in mice when chronically administered following a single dose of a carcinogen such as urethane. Evidence strongly indicates that the pulmonary effects of BHT are caused by one or more of its reactive metabolites, particularly the hydroperoxide or quinone methide products. The former, BHT-OOH (2,6-di-tert-butyl-4-hydroperoxy-4-methylcyclohexa-2,5-dienone+ ++), is later converted to free radicals by cytochrome P-450, and evidence implicating this pathway in BHT-OOH-induced cytotoxicity has been obtained using isolated rat hepatocytes. Pulmonary microsomes from mice effectively hydroxylate BHT to BHT-BuOH [6-tert-butyl-2-(hydroxy-tert-butyl)-4-methylphenol]; this metabolite was several-fold more effective than BHT as a lung tumor promoter, substantially more pneumotoxic than BHT in vivo, and more toxic to isolated rat hepatocytes and mouse bronchiolar Clara cells in vitro. These effects may be a result of oxidation of BHT-BuOH to the corresponding quinone methide, which is a highly electrophilic. The tumor promoting effects of BHT in mouse lung may be a result of selective cytotoxicity or altered signal transduction caused by radical-generating hydroperoxide and/or electrophilic quinone methide metabolites.

Free radical-derived quinone methide mediates skin tumor promotion by butylated hydroxytoluene hydroperoxide: expanded role for electrophiles in multistage carcinogenesis

Free radical derivatives of peroxides, hydroperoxides, and anthrones are thought to mediate tumor promotion by these compounds. Further, the promoting activity of phorbol esters is attributed, in part, to their ability to stimulate the cellular generation of oxygen radicals. A hydroperoxide metabolite of butylated hydroxytoluene, 2,6-di-tert-butyl-4-hydroperoxyl-4-methyl-2,5-cyclohexadienone (BHTOOH), has previously been shown to be a tumor promoter in mouse skin. BHTOOH is extensively metabolized by murine keratinocytes to several radical species. The primary radical generated from BHTOOH is a phenoxyl radical that can disproportionate to form butylated hydroxytoluene quinone methide, a reactive electrophile. Since electrophilic species have not been previously postulated to mediate tumor promotion, the present study was undertaken to examine the role of this electrophile in the promoting activity of BHTOOH. The biological activities of two chemical analogs of BHTOOH, 4-trideuteromethyl-BHTOOH and 4-tert-butyl-BHTOOH, were compared with that of the parent compound. 4-Trideuteromethyl-BHTOOH and 4-tert-butyl-BHTOOH have a reduced ability or inability, respectively, to form a quinone methide; however, like the parent compound, they both generate a phenoxyl radical when incubated with keratinocyte cytosol. The potency of BHTOOH, 4-trideuteromethyl-BHTOOH, and 4-tert-butyl-BHTOOH as inducers of ornithine decarboxylase, a marker of tumor promotion, was commensurate with their capacity for generating butylated hydroxytoluene quinone methide. These initial results were confirmed in a two-stage tumor promotion protocol in female SENCAR mice. Together, these data indicate that a quinone methide is mediating tumor promotion by BHTOOH, providing direct evidence that an electrophilic intermediate can elicit this stage of carcinogenesis.

Metabolism and cytotoxicity of eugenol in isolated rat hepatocytes

The metabolism and toxic effects of eugenol (4-allyl-2-methoxyphenol) were studies in isolated rat hepatocytes. Incubation of hepatocytes with eugenol resulted in the formation of conjugates with sulfate, glucuronic acid and glutathione. The major metabolite formed was the glucuronic acid conjugate. Covalent binding to cellular protein was observed using [3H]eugenol. Loss of intracellular glutathione and cell death were also observed in these incubations. Concentrations of 1 mM eugenol caused a loss of over 90% of intracellular glutathione and resulted in approximately 85% cell death over a 5-h incubation period. The loss of the majority of glutathione occurred prior to the onset of cell death (2 h). The effects of eugenol were concentration dependent. The addition of 1 mM N-acetylcysteine to incubations containing 1 mM eugenol was able to completely prevent glutathione loss and cell death as well as inhibit the covalent binding of eugenol metabolites to protein. Conversely, pretreatment of hepatocytes with diethylmaleate to deplete intracellular glutathione increased the cytotoxic effects of eugenol. These results demonstrate that eugenol is actively metabolized in hepatocytes and suggest that the cytotoxic effects of eugenol are due to the formation of a reactive intermediate, possibly a quinone methide.

Bioactivation of xenobiotics by prostaglandin H synthase

Prostaglandin H synthase (PHS) catalyzes the oxidation of arachidonic acid to prostaglandin H2 in reactions which utilize two activities, a cyclooxygenase and a peroxidase. These enzymatic activities generate enzyme- and substrate-derived free radical intermediates which can oxidize xenobiotics to biologically reactive intermediates. As a consequence, in the presence of arachidonic acid or a peroxide source, PHS can bioactivate many chemical carcinogens to their ultimate mutagenic and carcinogenic forms. In general, PHS-dependent bioactivation is most important in extrahepatic tissues with low monooxygenase activity such as the urinary bladder, renal medulla, skin and lung. Mutagenicity assays are useful in the detection of compounds which are converted to genotoxic metabolites during PHS oxidation. In addition, the oxidation of xenobiotics by PHS often form metabolites or adducts to cellular macromolecules which are specific for peroxidase- or peroxyl radical-dependent reactions. These specific metabolites and/or adducts have served as biological markers of xenobiotic bioactivation by PHS in certain tissues. Evidence is presented which supports a role for PHS in the bioactivation of several polycyclic aromatic hydrocarbons and aromatic amines, two classes of carcinogens which induce extrahepatic neoplasia. It should be emphasized that the toxicities induced by PHS-dependent bioactivation of xenobiotics are not limited to carcinogenicity. Examples are given which demonstrate a role for PHS in pulmonary toxicity, teratogenicity, nephrotoxicity and myelotoxicity.

Comparison of lung injury induced in 4 strains of mice by butylated hydroxytoluene

Butylated hydroxytoluene (BHT) is a phenolic antioxidant which induces lung injury in all strains of mice which have been tested, but not in any other species. The mortality of mice treated with BHT is also highly strain-dependent, with LD50s ranging from 138 to 1739 mg/kg. Despite this wide range of toxic doses, the relationship between lung damage and dose has not been well studied. The data presented here demonstrate that BALB/c, ICR and C57BL/6NHsd mice, with LD50s of 1739, 1243 and 917 respectively, exhibit similar time courses of repair (as assessed by the incorporation of radiolabelled thymidine into DNA) and pulmonary fibrosis (as assessed by lung hydroxyproline content) when given a single 400 mg/kg dose of BHT. SSIn mice, with an LD50 of approximately 350 mg/kg, also exhibited a similar time course of repair when given a single dose of 300 mg/kg BHT, although fibrosis did not develop in these animals. These data indicate that all strains of mice develop similar levels of lung injury at equivalent doses and that the extent of lung damage produced in mice does not correlate with the lethal dose.

Formation of glutathione conjugates during oxidation of eugenol by microsomal fractions of rat liver and lung

Rat hepatic and pulmonary microsomes catalyzed the formation of at least three distinct glutathione conjugates with eugenol (4-allyl-2-methoxyphenol). These three conjugates were identical with the products obtained from the chemical reaction of synthetic eugenol quinone methide and glutathione. The microsomal reaction was dependent on NADPH and oxygen and was inhibited by cytochrome P450 inhibitors such as metyrapone, 2-diethylaminoethyl-2,2'-diphenylvalerate (SKF 525-A), alpha-naphthoflavone and piperonyl butoxide. The enzyme responsible for eugenol oxidation was inducible with 3-methylcholanthrene but not phenobarbital pretreatment. The rate of formation of conjugates was not affected by the presence of glutathione-depleted cytosol which contained active glutathione transferase, even at low glutathione concentrations, suggesting that conjugation occurs nonenzymatically with an electrophilic metabolite of eugenol. Covalent binding to microsomal protein was observed using [3H]eugenol. Cumene hydroperoxide catalyzed the formation of these same glutathione conjugates via the formation of a quinone methide-like intermediate which was detected by spectroscopic means. Our results suggest that eugenol is oxidized by cytochrome P450 to a reactive quinone methide intermediate which can then covalently modify protein or conjugate with glutathione.

Evidence for a role of tert-butyl hydroxylation in the induction of pneumotoxicity in mice by butylated hydroxytoluene

Previous studies have shown that BHT must be biotransformed, probably to a quinone methide, in order to cause pneumotoxicity in mice. When BHT is incubated with mouse hepatic or pulmonary microsomes, a major metabolite that is formed is the tert-butyl-hydroxylated derivative of BHT (BHT-BuOH). Herein we show that BHT-BuOH has a fourfold greater potency than BHT in increasing the lung wt/body wt ratio, decreases lung cytosolic Ca2+-dependent protease activity at 1/10 the dose required for BHT to do this, and causes pulmonary histopathology at 1/20 the dose of BHT. Lung damage occurs earlier and is repaired faster at lower concentrations of BHT-BuOH than of BHT, but the nature of the damage (type I cell death) and regenerative response (type II cell hyperplasia and differentiation) is apparently identical. Neither BHT-BuOH nor BHT cause damage to liver, kidney, or heart as assessed by light microscopy, so they are both specific pulmonary toxicants. We postulate that BHT-BuOH formation is an essential step in the conversion of BHT to the ultimate pneumotoxin, which might be the corresponding quinone methide.

Enhancement of the peroxidase-mediated oxidation of butylated hydroxytoluene to a quinone methide by phenolic and amine compounds

We have recently demonstrated that butylated hydroxyanisole (BHA) markedly stimulates the peroxidase-dependent oxidation of butylated hydroxytoluene (BHT) to the potentially toxic BHT-quinone methide. Using both horseradish peroxidase and prostaglandin H synthase we now report the ability of a wide variety of compounds to stimulate peroxidase-dependent activation of BHT. These compounds include several phenolic compounds commonly present in pharmacologic preparations or occurring naturally in foods. The ability of a given compound to stimulate BHT oxidation was found to depend on the type of radical it forms upon peroxidase oxidation. Compounds which have been shown to form phenoxy radicals or nitrogen-centered cation radicals were observed to enhance BHT oxidation. Conversely, compounds which are known to form peroxy radicals or semiquinone radicals either inhibited or had no effect on BHT oxidation. Compounds which enhanced BHT oxidation (monitored by covalent binding of [14C]BHT to protein) were also observed to stimulate the formation of BHT-quinone methide and stilbenequinone. This suggested a common mechanism of interaction of these compounds with BHT. The stimulation of BHT covalent binding by BHA was also seen in various human and animal tissues using either arachidonic acid or hydrogen peroxide as substrate. The possible toxicologic implications of the enhancement of peroxidase-catalyzed BHT oxidation to BHT-quinone methide are discussed.

Enhancement of butylated hydroxytoluene-induced mouse lung damage by butylated hydroxyanisole

The phenolic antioxidant butylated hydroxytoluene (BHT) is known to produce a dose-dependent increase in mouse lung weight which is characterized by the necrosis of pulmonary type I and endothelial cells. We studied the ability of butylated hydroxyanisole (BHA) to modify BHT-induced changes in lung weight in male CD-1 mice. BHA alone had no effect on lung weight up to a dose of 500 mg/kg (sc). However, when injected 30 minutes prior to sub-threshold doses of BHT (0-250 mg/kg, ip), BHA significantly enhanced lung weight in a dose-dependent manner. The ability of BHA to enhance BHT-induced changes in lung weight was dependent on both the time and the route of administration of BHA relative to BHT. Deuteration of BHT abolished the in vivo toxicity from the combination of BHA and BHT. These results suggest that the toxicity resulting from the combination of BHA and BHT is due to the formation of BHT-quinone methide and that the role of BHA might be either to deplete some protective mechanism in the target pulmonary cells or to enhance the biotransformation of BHT into BHT-quinone methide.

Cytotoxicity of butylated hydroxyanisole and butylated hydroxytoluene in isolated rat hepatocytes

The effects of the antioxidants butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) on isolated rat hepatocytes were investigated. Both antioxidants were observed to be cytotoxic in a concentration-dependent manner at concentrations ranging from 100 to 750 microM. At equimolar concentrations BHT was more cytotoxic than BHA. Their toxicity appeared to be independent of their metabolism to reactive intermediates since inhibitors of cytochrome P-450 (metyrapone, SKF 525-A and piperonyl butoxide) had no effect on the cytotoxicity and N-acetylcysteine was also without protective effect. In addition, deuterated BHT was equitoxic with BHT. Only low temperature incubation (4 degrees), which has previously been shown to inhibit the insertion of these compounds into biomembranes, was effective in inhibiting the cytotoxic effects. Using isolated rat liver mitochondria we observed that both BHA and BHT inhibited respiratory control primarily by stimulating state 4 respiration and thus acting as membrane uncouplers. BHA and BHT also effectively dissipated membrane potential across the mitochondrial membrane and caused the release of calcium and mitochondrial swelling. These mitochondrial effects were reflected by a rapid decrease in ATP levels in intact hepatocytes which preceded cell death. These results suggest that the observed cytotoxicity of BHA and BHT to hepatocytes is related to their effects on biomembranes and mitochondrial bioenergetics.

Decreased pneumotoxicity of deuterated 3-methylindole: bioactivation requires methyl C-H bond breakage

The bioactivation of the pulmonary toxin 3-methylindole has been postulated to proceed via the formation of an imine methide. To test this hypothesis, the toxicity in mice of 3-methylindole has been compared to the toxicity of its perdeuteromethyl analog. Deuteration of the methyl group should slow the rate of production of the corresponding imine methide and diminish the toxicity of deutero-3-methylindole, if C-H bond breakage occurs prior to or during the rate-determining step. In agreement with this hypothesis, deutero-3-methylindole was synthesized and was shown to be significantly less toxic (LD50 735 mg/kg) than 3-methylindole (LD50 578 mg/kg). Both compounds produced the same lesion at the LD50 dose, bronchiolar damage and mild alveolar edema, indicating that deuteration of 3-methylindole did not change the pathologic process. However, at a much lower dose (25 mg/kg), 3-methylindole produced a mild bronchiolar lesion whereas deutero-3-methylindole did not damage lung tissue. Additionally, administration of deutero-3-methylindole caused less pulmonary edema compared to 3-methylindole, as assessed by increased wet lung weights. Finally, the depletion of pulmonary glutathione by deutero-3-methylindole was considerably slower than depletion by 3-methylindole. The electrophilic imine methide has been postulated to be the intermediate which binds with and depletes glutathione. Therefore, the evidence presented here supports the involvement of an imine methide as the primary reactive intermediate in 3-methylindole-mediated pneumotoxicity.

Hepatotoxicity of butylated hydroxytoluene and its analogs in mice depleted of hepatic glutathione

Butylated hydroxytoluene (2,6-di-tert-butyl-4-methylphenol, BHT) has been reported to be a lung toxicant. Mice treated with BHT (200-800 mg/kg, po) in combination with an inhibitor of glutathione (GSH) synthesis, buthionine sulfoximine (BOS; 1 hr before and 2 hr after BHT, 4 mmol/kg per dose, ip) developed hepatotoxicity characterized by an increase in serum glutamic pyruvic transaminase (GPT) activity and centrilobular necrosis of hepatocytes. The hepatotoxic response was both time- and dose-dependent. BHT (up to 800 mg/kg) alone produced no evidence of liver injury. As judged by the observation of normal serum GPT, drug metabolism inhibitors such as SKF-525A, piperonyl butoxide, and carbon disulfide prevented the hepatotoxic effect of BHT given in combination with BSO. On the other hand, pretreatment with cedar wood oil resulted in increased hepatic injury in mice treated with both BHT and BSO. Pretreatment with phenobarbital also tended to increase hepatic injury as judged by changes in serum GPT. These results suggest that BHT is activated by a cytochrome-P-450-dependent metabolic reaction and that the hepatotoxic effect is caused by inadequate rates of detoxification of the reactive metabolite in mice depleted of hepatic GSH by BSO administration. The hepatotoxic potencies of BHT-related compounds also were examined in BSO-treated animals. For hepatotoxicity, the phenolic ring must have benzylic hydrogen atoms at the 4 position and an ortho-alkyl group(s) that moderately hinders the hydroxyl group. These structural requirements essentially are the same as those for the toxic potency in the lung (T. Mizutani, I. Ishida, K. Yamamoto, and K. Tajima (1982), 62, 273-281) and support the hypothesis that BHT-quinone methide plays a role in producing liver damage in mice with depressed hepatic GSH levels.

The toxicological implications of the interaction of butylated hydroxytoluene with other antioxidants and phenolic chemicals

Butylated hydroxyanisole (BHA) enhanced both the in vitro peroxidase-catalysed covalent binding of butylated hydroxytoluene (BHT) to microsomal protein and the formation of BHT-quinone methide. Eugenol, methylparaben, vanillin, guaiacol, ferulic acid and several other phenolic compounds commonly used in food and cosmetic products also enhanced the metabolic activation of BHT. BHA was the most effective compound tested. Microsomes from lung, bladder, kidney medulla and small intestine of various animal species, including man, were also able to support this interaction of BHA and BHT using either hydrogen peroxide or arachidonic acid as the substrate. These in vitro observations were extended to an in vivo mouse lung model. Subcutaneous injections of BHA significantly enhanced the lung/body weight ratio of mice given intraperitoneal injections of subthreshold doses of BHT. The toxicological implications of the interactions of BHT with other antioxidants and phenolic chemicals and their potential relevance to human risk are discussed.

Protection by methylprednisolone against butylated hydroxytoluene-induced pulmonary damage and impairment of microsomal monooxygenase activities in the mouse: lack of effect on fibrosis

The effects of the synthetic corticosteroid methylprednisolone (MP; 30 mg/kg, s.c. given twice daily for 3 days), on the pneumotoxic effects of a single dose of butylated hydroxytoluene (BHT; 400 mg/kg, i.p.) over a 10 day experimental period was investigated in male C57BL/6N mice. BHT alone caused time-dependent alveolar hypercellularity, inflammatory infiltration, alveolar septal thickening and hypercellularity of the bronchiolar epithelium, reaching a maximum by day 5 with some degree of recovery by day 10. The pulmonary monooxygenase activities reflected the degree of alveolar damage and Clara cell abnormality with time; reductions in monooxygenase activities occurred and minimum levels (7-15% of control) were reached by day 5 and again a trend towards recovery by day 10. MP administered 0, 24 and 48 hr after BHT treatment partially protected mice from these effects of BHT in a distinctly time-dependent fashion; the degree of protection decreased as the time between BHT challenge and MP treatment increased. Although MP alone did not morphologically affect Clara and alveolar cells, it increased, decreased or had no effect on the monooxygenase activities. About 25% of the mice that received BHT alone died by day 5 and 50% by day 10. MP completely blocked the lethal effects of BHT by day 5 and reduced the deaths to between 15% and 25% by day 10. Interestingly, MP did not protect against the BHT-induced pulmonary fibrosis, measured as total lung hydroxyproline content, irrespective of the time between BHT challenge and MP treatment. MP alone did not cause any deaths nor increase lung hydroxyproline content.

Procarbazine spermatogenesis toxicity: deuterium isotope effects point to regioselective metabolism in mice

Procarbazine was shown to decrease spermatogenesis in male mice in a dose-dependent manner. Significant decreases (44% of controls) in spermatogenesis were observed when a dose of 400 mg/kg was administered 18 days prior to determination of sperm count. Procarbazine caused no significant acute spermatocidal activity in vivo. Procarbazine-associated decreases in spermatogenesis were thus used as an index of toxicity to developing spermatid cells. Procarbazine analogs were synthesized that had deuterium substituted for hydrogen at the benzylic position, N-isopropyl-alpha-(2-methylhydrazino)-p-[alpha, alpha-2H2]toluamide (d2-procarbazine), or at the methyl position, N-isopropyl-alpha-(2-[alpha, alpha, alpha-2H3]methylhydrazino)-p-toluamide (d3-procarbazine). Spermatogenesis decreases caused by d3-procarbazine were essentially the same as with procarbazine in mice (66% of controls at a dose of 200 mg/kg), but d2-procarbazine was nontoxic to developing sperm cells (99% of control at a dose of 200 mg/kg). The decrease in toxicity caused by deuterium substitution at the benzylic position, coupled with the absence of an effect with the methyl-labeled analog, indicate the requirement for regioselective oxidative metabolism of procarbazine at the benzylic position prior to the toxic event.

2-Acetamido-p-benzoquinone: a reactive arylating metabolite of 3'-hydroxyacetanilide

The covalent binding to protein of 3'-hydroxyacetanilide (3HAA), its primary metabolite 2',5'-dihydroxyacetanilide (2,5DHAA), and a putative secondary metabolite thereof, 2-acetamido-p-benzoquinone (APBQ), was studied in hepatic microsomal preparations from phenobarbital-pretreated mice. All compounds were found to bind irreversibly to microsomal protein, APBQ being by far the most effective member of the group. In the case of 3HAA, binding was dependent upon the presence in incubation media of the co-factor NADPH, indicating that metabolism of 3HAA was necessary for the generation of a reactive intermediate. In contrast, NADPH decreased by more than 2-fold the binding of both 2,5DHAA and APBQ. The free radical spin-trapping agent alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (POBN) did not reduce the binding of 3HAA to protein. These results support the contention that metabolic activation of 3HAA is a two-step process which involves initial aromatic hydroxylation to give the substituted hydroquinone, 2,5DHAA, followed by a second oxidation reaction (which may not be enzyme-mediated) to produce the benzoquinone derivative, APBQ. This quinone is a reactive, electrophilic intermediate which may either undergo reduction back to 2,5DHAA or bind covalently to cellular macromolecules.

Systematically applied chemicals that damage lung tissue

A large, and increasing number of drugs and chemicals have been found which are toxic to lung following systemic administration. These agents damage lung tissue specifically, or in addition to damage to other tissues. Mechanisms explaining the pulmonary damage produced by some lung toxins have been uncovered. These include concentration of the agent within lung, the absence of adequate pulmonary detoxication systems, and bioactivation to a toxic species within specific lung cells or at distant sites followed by transport to the lung. The basic biochemical lesions underlying lung damage, responses of individual lung cells and pulmonary repair processes to the toxic agent, and species and age differences in susceptibility to lung damage have not, however, been well defined for most lung toxins. This review describes the information available on pulmonary biochemical and pathological changes associated with some of these lung-toxic agents. In addition, mechanisms proposed to explain the lung damage are discussed. The agents covered include: paraquat, the thioureas, butylated hydroxytoluene, the trialkylphosphorothioates, various lung-toxic furans and antineoplastic agents, the pyrrolizidine alkaloids, metals and organometallic compounds, amphiphilic agents, hydrocarbons, oleic acid, 3-methylindole, and diabetogenic agents. Detailed reviews on the overall toxicity of many of these agents have been published elsewhere. This review concentrates on their pulmonary toxicity. Information is presented as an overview to illustrate both the extensive literature that is available and the important questions that remain to be answered about systemic chemicals that damage lung tissue.

Formation of a glutathione conjugate from butylated hydroxytoluene by rat liver microsomes

Butylated hydroxytoluene (BHT) was converted to S-(3,5-di-tert-butyl-4-hydroxybenzyl)-glutathione (BHT-glutathione) by rat liver microsomes in the presence of NADPH, molecular oxygen, and glutathione. NADH was far less effective than NADPH and exhibited little synergistic effect when used together with NADPH. Cytochrome P-450 inhibitors, such as SKF 525-A, alpha-naphthoflavone, metyrapone, and carbon monoxide, significantly inhibited BHT-glutathione formation. Liver microsomes from phenobarbital-treated rats catalyzed the formation of BHT-glutathione at a rate that was nine times the rate of adduct formation by control microsomes. No stimulation of BHT-glutathione formation was observed with the addition of liver cytosol fraction to the microsomal incubation mixtures even at low glutathione concentrations. These results support the view that BHT is converted by the cytochrome P-450 monooxygenases to a chemically reactive metabolite, possibly BHT-quinone methide, which forms BHT-glutathione by nonenzymatic conjugation with glutathione.

Modification of butylated hydroxytoluene-induced pulmonary toxicity in mice by diethyl maleate, buthionine sulfoximine, and cysteine

Treatment of mice with diethyl maleate (DEM) or buthionine sulfoximine (BSO) significantly enhanced the lung injury caused by butylated hydroxytoluene (BHT). Conversely, cysteine protected mice from the lung toxicity of BHT. BHT administration to mice produced a time-dependent reduction of glutathione (GSH) content in the lung, but not in the liver. These results support the concept that conjugation of 2,6-di-tert-butyl-4-methylene-2,5-cyclohexadienone (BHT-quinone methide), a proposed reactive metabolite of BHT, with GSH is involved in the detoxification of BHT in mice.

Prevention of butylated hydroxytoluene-induced lung damage by diethyldithiocarbamate and carbon disulfide in mice

Diethyldithiocarbamate (DTC) and carbon disulfide (CS2), at nearly equimolar doses (po), prevented mice from lung injury induced by butylated hydroxytoluene (BHT), as evidenced by suppression of increased lung weight and total DNA content as well as by histopathologic observations. CS2 pretreatment dose dependently decreased the amount of covalently bound [ring-14C]BHT to lung macromolecules in vivo. A slight, but significant, loss of lung GSH observed early after BHT administration was also prevented. The lung microsomal fraction exhibited NADPH-dependent covalent binding of BHT in vitro; this was inhibited completely by carbon monoxide and slightly by SKF-525A. This NADPH-dependent binding was suppressed in lung microsomes isolated from CS2-treated mice. CS2 also reduced various drug metabolizing enzyme activities and the cytochrome P-450 content of the lung microsomal fraction. These results support the metabolic activation hypothesis for BHT-induced lung damage, and the preventive action of CS2 and DTC may be due to an inhibition of this bioactivation step. Possible sites of the metabolic activation of BHT and its inhibition by CS2 are discussed.